In order to monitor and control a process, it is useful to identify parameters that accurately and repeatedly measure the status of the process. In the case of processing photographic film, it is well known to describe the photographic response of each particular film to that process by a curve. This curve is typically referred to as the “characteristic curve” for the film and it represents the relationship between developed density of the photosensitive emulsion on the film and the logarithm of exposure of the emulsion to light. This curve is often referred to as the H & D curve, named after Hurter and Driffield, The Journal of the Society of Chemical Industry, No. 5, Vol. IX, May 31, 1890.
The “characteristic curve” is determined using a control strip as is well known in the art. The control strip is produced by taking a small piece of film and exposing it in a sensitometer by contact with an original step wedge, which has, typically, 21 densities in steps of 0.15 log exposure units (for X-ray films, for example), with light of a color appropriate to the type of film being used for process control (typically either blue or green for X-ray films). The exposed strip is processed in the processor whose performance is being monitored, and is then ready to be measured.
The great majority of motion picture film processing laboratories use sensitometry extensively to monitor and evaluate the quality and consistency of various variables affecting their film processing. Sensitometric quality control procedures used by these laboratories typically entail processing pre-exposed film control strips and then measuring the red, green and blue densities of these processed control strips. The measured densities are then compared with the densities evinced by reference control strips provided by the film manufacturer. Various process control variables may then be adjusted, if necessary, to improve and/or correct the processing of the film, according to principles well known in the art.
Sensitometry requires that the photographic emulsion on the test strips be exposed to a specified light source for a specified time and then the film processed in closely controlled conditions. The resultant densities produced on the test film are then measured and plotted against a typically logarithmic exposure scale. The most common method for determining the effect of exposure and processing on a sensitometric strip is to measure its light stopping ability. As illustrated in FIG. 1, when incident light 10 strikes a photographic film 20, a portion 30 of the incident light is reflected backwards, the grains of the silver halide emulsion 40 on the film 20 absorb another portion 50, and most of the remainder 60 of the light is scattered as a result of bouncing off the grains of the emulsion. The light stopping ability of a film is a combination of these three effects, and is typically denoted in terms of its transmittance.
Transmittance is defined as the ratio of transmitted light to the incident light:
      Transmittance    =            Transmitted Light              Incident Light      
With reference now to FIG. 2, in an example where 100 units of light 10 are incident on a film 20 and 50 units of light are transmitted therethrough, the transmittance of the film is equal to 50/100=0.5. The numerical value of the transmittance becomes smaller as the light stopping ability increases, making numerical precision somewhat cumbersome. Thus, it is sometimes preferable to refer to the opacity O of a film, which is defined as the ratio of incident light to the transmitted light:
  O  =                    Incident Light                    Transmitted Light              =          I      T      
The opacity of a film increases in geometric proportion with the film thickness and hence another term called density is commonly used to express the photographic effect of a film. The concept of density is illustrated in FIG. 3, and is defined as
      Density    =            log      ⁡              (        O        )              =          log      ⁢              I        T            
The concept of density provides a numerical description of the image that is a more useful measure of the light stopping ability of a film. Additionally, the human eye has a nearly logarithmic response to an image and hence density values more appropriately represent the description of such an image.
To correctly measure density it is necessary to measure the units of transmitted light 10. The transmitted light rays 10 are grouped in a certain distribution as a result of bouncing off the emulsion grains 40. This distribution of transmitted light will be wider for coarse-grained images than for fine-grained images because the larger grain size provides a greater surface area over which bouncing can occur. As a result, coarse-grained images scatter more light than fine-grained image.
With reference to FIG. 4, when a photoreceptor 70 is placed far from a film sample 20, only light transmitted over a very narrow angle will be recorded in what is commonly called specular measurement. Alternatively, when the photoreceptor is placed in contact with the film sample, all of the transmitted light will be collected because the angle of collection is much larger. This is commonly referred to as diffuse measurement.
The relationship between the diffuse density and the specular density for a given sample is called the Callier co-efficient, or Q factor, and is defined as
  Q  =            Specular Density              Diffuse Density      
The actual conditions of density measurements vary with the purpose for which these values are to be used. If the purpose is to predict the printing characteristics of the negative, then the spectral response characteristic of the print film should be simulated. To determine the visual appearance of the image, the spectral response of the human eye should be simulated. In the first case the result is called the printing density and in the second case the result is called the visual density. If the conditions of measurement do not simulate the photographic system being used, the resulting data will lack the validity even though sophisticated, well calibrated instruments are used.
To evaluate and understand the results of the sensitometric tests discussed above, it is necessary to plot the densities occurring on the test strip in relation to exposures to which the film was subjected to produce each such density. The characteristic curve obtained is called, variously, either a D log(E) curve, an H and D curve, or a log(It) curve. In this curve density is represented on the vertical (Y) axis of the graph and the logarithmic values of the exposure or the log It (Intensity×Time) are represented on the horizontal (X) axis of the graph.
To obtain a characteristic curve for a particular film, a sample of that film is exposed to a light source in a sensitometer by using either a Time scale or an Intensity scale. In the Time scale approach the length of time of exposure is varied, whereas in the Intensity scale method the current is changed so as to vary the light intensity of the sensitometer. A film exposed in a sensitometer produces what is commonly referred to as a step wedge (see FIG. 6).
There are three common types of photographic step wedges that are commonly used: the three patch wedge, the 11 step wedge, and the 21 step wedge (also referred to as a √2 wedge). Each of these wedges have particular benefits, but the 21-step wedge shown in FIGS. 5 and 6 gives the best results as it gives a smoother, more accurate curve. The reason it is also referred to as a √2 wedge is that the difference between each exposure or step in the wedge is equal to the previous exposure multiplied by √2 or 1.414. This fits on to the log(It) scale very well because the log value of √2 is 0.15, as illustrated in FIG. 5.
Referring to FIG. 7, the characteristic curve thus plotted can be conveniently divided into four major sections: base plus fog, toe, straight line, and shoulder. The base plus fog region represents the combination of the density of the emulsion support (base) and the density arising from the development of some unexposed silver halide crystal (fog). Here the curve is horizontal and the film is not capable of recording subject details or tonal differences. The toe region is characterized by low density and constantly increasing slope as exposure increases. It is in this area that shadow details in the subject are normally placed.
With reference to FIG. 8, the straight line region is the middle density region where the slope (also called gamma) is nearly constant and is steepest. It is in this region that subject tones are reproduced with greatest separation, and this is therefore the most useful section of the film. The shoulder is the portion where the density is high but the slope is decreasing with increase in exposure. Most of this section is usually avoided when exposing film.
Sensitometry is in wide use and has been the subject of a number of attempts to improve upon it. In U.S. Pat. No. 4,508,686 an apparatus and test strip for evaluation of a film processor are disclosed. The apparatus evaluates the optical density of graded density test areas on a developed (processed) film by comparing a photodetector signal with a preselected voltage relating to an acceptable/too dark threshold of an unexposed or base fog area, a maximum density or dark area, and a medium density area. This method thus also relies on separate test strips to evaluate the performance of a film processor at timed intervals.
Another approach is described in U.S. Pat. No. 4,985,320 and entails using a voltage set point system to provide a constant illumination of a photographic test strip and a voltage divider comparator network for accurately determining exposed film density levels. The method thus provides an indication of the state of the film developer solution that is substantially independent of the temperature of the photodetector or large changes in the intensity of the test light source. Similarly, U.S. Pat. No. 4,004,923 describes a method for controlling developer activity by exposing a test film having various transparent areas and opaque areas, and insets and background areas that can blend into one another when the developer fluid is fresh. These methods therefore also rely on the use of a test strip exposed at predetermined time intervals.
U.S. Pat. No. 5,481,480 proposes a novel formula to describe the characteristic curve of a material as assessed with a step wedge as described above. The characteristic curve expression takes into account the density at saturation as well as certain constant parameters for the particular material. Therefore, this method also does not obviate the need for a separate film test strip for obtaining a step wedge.
Sensitometry procedures as currently known and utilized in motion picture film processing laboratories involve measurement of pre-exposed control film strips at fixed time intervals. However, film processing machine speeds have increased significantly in recent years, thereby resulting in a lesser frequency of sampling due to the fixed time intervals at which sampling is conducted. As illustrated in FIGS. 9 and 10, at a frequency of one sample per hour, the frequency of sampling per length of film drops significantly with the amount of film processed per hour. What is therefore now needed are improved methods and apparatuses for assessing process quality control variables suitable for use with modern, high speed film processing machines. The embodiments disclosed herein address this and other needs.